Transmission arrangements for transmitting a useful signal modulated onto a carrier oscillation, with a bandpass filter wired into the transmission path of the useful signal in the transmission arrangement, and tuned to the frequency of the carrier oscillation, are known especially from radio or broadcast technology or data technology. These useful signals are designed to be transmitted so that at the reception location, they deliver at least somewhat reliable information for further processing of the data, or for reproduction of an audio or video depiction.
In the transmission of amplitude-modulated useful signals, for example audio signals in the shortwave region, fading occurs especially at night or over long broadcast distances; this distorts the transmitted useful signal, and superimposes interference noise onto it, to such as extent that the information in this useful signal, for example speech and music, is understandable or recognizable only with difficulty, or at times not at all. The same applies to data signals transmitted in this manner. However, in large and sparsely populated regions of the world the only economical way in which to disseminate news and/or entertainment broadcasts is by means of amplitude-modulated useful signals.
On the other hand, the transmission of frequency-modulated useful signals also raises problems of highly variable reception field strengths, especially in heavily built-up or mountainous regions. At very low field strengths considerable interference occurs especially in the outer regions of the sidebands--i.e. in the higher-tone region for an audio frequency transmission--causing severe noise particularly in these regions.
In the transmission of frequency-modulated useful signals that have previously been recorded on videotape, an undesirable loss of information results from the fact that the upper sideband is almost completely cut off. To restore this sideband at least partially, a patent application DE 40 35 309.5--unpublished at the time this invention was filed--proposes to arrange in the transmission path of the useful signal an amplitude-controlled delay element that changes the phase of the individual oscillations as a function of the amplitude of those oscillations, thereby producing an oscillation that contains both the lower and the upper sideband as information. This application also proposes that in a soft circuit transition, a relatively narrow-band bandpass filter be inserted into the transmission path, instead of the delay element if it is no longer possible to guarantee fairly good demodulation of the transmitted useful signal. For this purpose, the proposed distortion correction circuit contains an error detector which recognizes, and classifies as errors, instances in which expected oscillations are missing or are transmitted at such low amplitude that they do not reach a threshold set in the error detector; and which generates an error signal characterizing the frequency with which errors occur. Soft switching from delay mode to bandpass filter mode then occurs on the basis of error frequency, in such a way that as the error frequency rises, bandpass filter mode is already activated at low signal amplitudes. With the proposed features, not only is the more suitable of two selectable distortion correction arrangements selected on the basis of signal amplitude, but the selection boundary is also defined as a function of error frequency.